Solid propellant rocket motor cases for missile systems, spacecraft boosters and other types of large and small high performance, lightweight pressure vessels are commonly made from fiber reinforcement and various formulations of polyepoxide resins (epoxy resins) by a filament winding process. Similarly, filament winding with both polyesters and epoxy resins has made possible production of lightweight tanks, poles, piping and the like. Historically, fiberglass has been the most common reinforcement fiber, but other fibers such as carbon filaments, boron filaments, and high modulus organic polymer filaments, most significantly aramid filaments, have become increasingly useful in these composite structures to take advantage of their differing and sometimes unique physical properties.
The resins utilized are typically epoxy formulations based on diglycidyl ether-bisphenol A (DGEBA), reactive low molecular weight epoxy diluents and curing agents such as aliphatic and aromatic amines and carboxylic acid anhydrides. Both flexibilized and rigid epoxy resins have been used as matrix resins for filament wound composite structures.
In providing composite articles, such as the aforesaid pressure vessels, one has employed either a wet winding process or a prepreg process. The resin-fiber combination is to be employed in wet winding, the fiber is simply run through a resin bath containing the resin composition whereby the fiber is coated with the composition. The resulting resin-fiber combination is then wound directly into the desired structure, which is then cured by polymerization by means of heat or radiation. On the other hand, if a prepreg is to be used, the fiber or "tape" is impregnated with a curable resin composition and then wound on a spool as a prepreg and stored for winding at a future time. When the prepreg is converted into a composite article, the prepreg is then typically cured by polymerization by means of heat or radiation.
The present invention relates to matrix resin formulation especially suitable for and useful as prepreg compositions. A prepreg is composed of a reinforcing fiber and a curable resin matrix and is generally in one of the forms referred to as tow, roving, tape, mats, fabric and the like. In the past, preparation and use of prepreg materials has been time consuming and expensive, especially for long-working-life prepreg. By long-working-life prepreg is meant a prepreg whose handling properties do not change significantly over thirty days in normal room handling conditions.
In order to obtain and use such long-working-life prepreg, constraints at four stages in the processing sequence must be taken into consideration, namely, at the following stages: impregnation and spooling, filament winding or lay-up, cure minimum and ultimate cure of the composite article.
During impregnation the resin formulation must have a viscosity low enough so that it will penetrate fiber bundles containing many thousands of filaments, thoroughly and evenly wetting the individual filaments. Viscosities are typically under 5,000 centipoise (cps). Spooling requires high enough viscosity so that the resin does not squeeze out as the fiber is spooled. A nominal spooling viscosity for graphite fibers is generally about 1,000 cps.
Two constraints operate on resin viscosity during filament winding or lay-up. The resin must have low enough viscosity so that the prepreg conforms to the surface, minimizing interlaminar voids. Resin viscosity must be high enough, however, that minimum viscosity during cure does not go below about 500 cps. While these constraints leave a broad range for acceptable viscosities, the cure minimum of 500 cps precludes the use of heat-cured resins whose room temperature viscosity is 5,000 cps or less. A resin with a room temperature viscosity of 5,000 cps would fall far below 500 cps during heated cure. In consequence, viscosity must rise between spooling and use.
Techniques used to cure a matrix material often temporarily reduce its viscosity. Heated cure of typical epoxy prepreg resins can reduce their viscosity by several orders of magnitude for an hour. If viscosity falls too low, matrix material bleeds from a curing part, compromising its quality. While it is important that the matrix viscosity not fall too low during cure, it is also typically important that it becomes liquid. Failure of the matrix material to melt can be the source of void and delamination defects in composite parts.
After the cure minimum, the chemical reactions involved in curing a matrix resin progress, progressively raising the viscosity to gellation or to the level required for use of the composite part.
Heretofore, two techniques or processes have been employed to prepare long-working-life prepregs, namely, solution dilution impregnation and hot-melt impregnation. In solution impregnation a matrix resin with a viscosity of greater than 5,000 cps is diluted with a solvent to a viscosity of less than 5,000 cps. The fiber is impregnated with this diluted resin, then solvent is removed by heating and evaporation before the prepreg is spooled. Problems with this approach include the ecological requirement that the solvent be recovered, the associated expense and the inevitable residual solvent in the matrix resin. In hot-melt impregnation a matrix resin with a room temperature viscosity greater than 5,000 cps is heated to a temperature where its viscosity is less than 5,000 cps. Fiber is impregnated with the matrix resin at that temperature, the prepreg is cooled, then spooled. Problems with this approach include the need for matrix heating equipment, and rising viscosity of the matrix resin due to heat-induced polymerization during impregnation. Moreover, after the resin-fiber prepreg has been spooled, in each of these processes, the prepreg must generally be stored under refrigerated conditions to prevent the prepreg from going to ultimate cure which would prevent its use in winding or forming composite articles.
It would therefore be most desirable if a matrix material or resin formulation could be provided that would go through the aforedescribed desired viscosity profile at room temperature and do so without requiring solvent dilution or hot-melt impregnation of fibers. It would also be desirable for one to be able to spool the prepreg at room temperature substantially immediately after impregnation of the fiber and without requiring either solvent removal or cooling of the prepreg. It is also desirable that, after impregnation, the viscosity of the matrix rises and then levels off at a viscosity level planned for room temperature storage and later use of the prepreg thereby allowing for a long term room temperature storage of the prepreg and also for a long-working-life. It is still further desired that the prepreg, when the prepreg is used to form a composite article, goes to a viscosity minimum and then gels, cures or hardens like a typical prepreg. Also, it is desirable that the matrix processing viscosity be controlled by chemical formulation rather than by solvents or heated impregnation equipment.